Help needed with frame to cab mount brackets 69 cab on 65 frame

[tranz am]

I have been working on a custom frame for my 69 GMC short bed that you can see in my build thread in my signature. the frame that I've used is a 1965 c10 short bed. I was told the frames were the same 62-72 and they appear to be the same with only slight differences.
Anyways I have determined one of these differences is the rear cab mount bracket, which seems to be an inch taller than the one used on the 69. I had a spare frame from a 68 which we had previously cut up and i have the section where the rear cab mount bracket normally is plug welded to.
the problem that I am encountering now is that none of the holes seem to line up. There is a big hole that I believe to be a positioning hole. my question is do I use this hole to center my 69 cab mount bracket? here are some pics. Any insight with this is apreciated.

[tranz am]

I am having difficulty accesing my photobucket right now so cant post the other picture but when overlapped none of the bolt holes line up when centered on the big alignment hole.

[tranz am]

I will try again later to get the picture up.

[tranz am]

[tranz am]

Bump.

[tranz am]

So basicly what I am trying to figure out is if the larger hole in the center is in the same location on the early frame as the 67-72. If so then this will solve my problem.

Correct me if I am wrong but I believe the front mount brackets are the same and in the same location on both generations.

I need to sleeve the hole needed for the mount bracket in the boxed in section of the frame before I plate the inside.

[LSUMurse]

I'm sure you've attempted something like this, but is there something on both frames that you KNOW is the same? Use that as a reference point and measure to your hole.

(Sorry if that is completely impossible. I've never done something like what your doing, just seems plausible.)

[WIDESIDE72]

Unfortunately, imo you are on your own on this one. Probably not too many folks have a "backdated" frame swap. Good luck, sorry i cant help.

[tranz am]

I have another frame to measure of off but its currently in use, and a little more difficult to measure being that the holes are still plug welded and the mount brackets are in the way . I might be able to make some sort of jig to measure the dottom holes from front to back, being I am pretty sure the front holes are in the same location on both frames.

I am building this frame on the side until its ready for the 69 body/cab swap. I want my truck to be mobile as long as it can be.